There are several techniques for speciation analysis that rely on elemental detection using atomic spectrometry, along with a separation technique to differentiate between different species. For liquid samples, liquid chromatography is the most commonly applied separation technique, while Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is the most frequently used detection method. However, the conventional sample introduction system for ICP-MS, which involves a pneumatic nebulizer in a spraychamber, results in poor sample transfer efficiency.
Sample introduction has been identified as the "Achilles' heel" of atomic spectrometry. Many efforts have devoted to improve sample transfer efficiency and with it the detection power. Chemical vapour generation (CVG) is an important sample introduction technique, and is widely used for atomic spectrometry for its enhanced sensitivity and selectivity. Non-volatile analytes can be transformed to volatile or semi-volatile species through chemical reaction. The generated gaseous analyte compounds can be separated from the sample matrix by a gas-liquid separator and subsequently transferred to the detector i.e. ICP-MS for detection, largely eliminating spectral and non-spectral interferences caused by matrix elements.
Apart from these advantages, chemical vapour generation also has some drawbacks. Chemical vapour generation efficiency depends on the analyte species being present. For conventional trace element analysis this problem can be circumvented by appropriate sample preparation, transforming all element species into a single species. However, this approach cannot be used for speciation analysis, for which analyte transformation during sample preparation has to be avoided.
Analyte species transformation then has to be done after separation of species on-line on the way to the detection system. Unfortunately adding reagents post-column to the eluate and pushing the eluate through a reactor for the species transformation is adding dispersion to the sample flow, reducing the sensitivity gain obtained by enhanced analyte transfer through vapor generation as well as the separation power by peak broadening.
On the other hand, the vapour generation of only selective species can be used for an operationally defined fractionation of those species that can be volatilized against those than are not volatilized. Such selective vapour generation can then be used for binary speciation without any prior species separation by chromatography. In general, selectivity can be controlled to some extent by selecting reaction parameters such as pH and derivatizing agent. Examples of such methods are the determination of inorganic/organic mercury or inorganic/organic arsenic. The main advantage of such non-chromatographic speciation methods is the much simpler instrumentation and the higher sample throughput.
Michael Sperling
Related Reviews of the technique
Vapor generation for sample introduction
Mariusz Slachcinski,
Modern chemical and photochemical vapor generators for use in optical emission and mass spectrometry, J. Anal. At. Spectrom., 34 (2019) 257-273.
DOI: 10.1039/c8ja00383a
Selective vapor generation for binary speciation analysis:
Maja Welna, Anna Szymczycha-Madeja, Pawel Pohl,
Non-Chromatographic Speciation of As by HG Technique—Analysis of Samples with Different Matrices, Molecules, 25 (2020) 4944; doi:10.3390/molecules25214944
Chemical vapor generation as an interface for coupling HPLC and atomic spectrometry
Yasin Arslan, Emrah Yildirim, Mehrdad Gholami, Sezgin Bakirdere,
Lower limits of detection in speciation analysis by coupling high-performance liquid chromatography and chemical vapor generation, Trends in Analytical Chemistry, 30/4 (2011) 569-585. DOI: 10.1016/j.trac.2010.11.017
last time modified: November 12, 2024
